In conventional spectrometers, a beam of spectral energy composed of the separate wavelengths to be analyzed, is passed through a dispersive element to disperse the band into a spectrum wherein the separate wavelengths are spatially spread out. A slit is used to pass only a narrow band of the wavelengths to a detector and the individual wavelengths are analyzed or scanned by mechanically moving either the dispersive element or the slit. The slit needs to be relatively narrow to achieve a fine resolution and the energy contained within the bandwidth passed by the slit is relatively small in comparison to the energy of the whole spectrum being scanned. The detector thus measures a relatively small signal so that the signal-to-noise ratio is relatively low thereby decreasing the spectrometer sensitivity.
Described in "Hadamard-Transform Analytical Spectrometer" by John A. Decker, Jr., "Analysis Instrumentation - Volume 10", Instrument Society of America, pages 49-54 and in U.S. Pat. No. 3,578,980 -- Decker, Jr. et al. is a Hadamard-Transform Spectrometer (HTS). The HTS is an analytical instrument useful in both the visible and the infrared spectral regions and performs Fourier-Transform interferometric spectrometry functions while using the simple technology of conventional dispersive spectrometers. The HTS is a multiplex instrument, which means that it observes all the wavelengths in a spectrum at the same time and hence has a multiplex advantage of a high signal-to-noise ratio over conventional scanning monochromator spectrometers. The HTS is also a transform instrument, so that multiplexing is accomplished through an optical coding process, and the measured data points are the mathematical transform of the inputted dispersed optical spectrum. The HTS uses conventional dispersive optics (prisms, gratings, etc.) for spectral discrimination, and is not an interferometric instrument.
Hadamard spectroscopy is implemented by placing a series of masks at the exit or entrance slit of a conventional dispersion spectrometer. For each mask a different combination of spectral elements falls on the detector. Intensities measured with N different masks can be used to compute the intensities of N different spectral elements. This can be represented by a set of simultaneous linear algebraic equations represented in matrix notation as follows: EQU (a.sub.ij) [x.sub.j ] = [I.sub.i ]
i = 1 - - - - -N PA1 j = 1 - - - - -N
Here x.sub.j denotes the intensities of the desired spectral elements and I.sub.i is the combined measured intensity. The elements of the coefficient matrix [a.sub.ij ] may be chosen such that the above equations can be inverted and solved for [X.sub.j ] when the I.sub.i values are measured. Thus, by a proper choice of transparent and opaque elements along the elements of the encoding masks, it is possible to deduce the intensity of the spectral radiation through each such element from sequential measurements of the radiation intensity reaching a detector. Instead of N masks, a single mask may be used having (2N- 1) elements. The above article and patent describe further details of the prior art HTS.
An HTS of the above type is disadvantageous because it is relatively slow due to the need to either replace or substitute one mask for the other or to step along a single mask between the various positions and, in addition, requires the use of a relatively costly mechanism for precisely locating the mask or moving the single mask.